fix: Set Brand.default_application so "/" doesn't dead-end at /if/user/

Several dead ends (TOTP/WebAuthn setup after an anonymous visit bounces
through login and lands on "/") trace back to Brand.default_application
being unset, which falls back to /if/user/ - blocked for type=external
Matrix accounts. Set it to the matrix Application. Only changes the bare
"/" fallback; explicit URLs like /if/admin/ are unaffected.

apps/authentik/authentik-blueprints.yaml

@@ -206,3 +206,23 @@ data:
           order: 10
         attrs:
           stage: !Find [authentik_stages_redirect.redirectstage, [name, matrix-invitation-redirect]]
+  matrix-brand-default-app.yaml: |
+    # yaml-language-server: $schema=https://goauthentik.io/blueprints/schema.json
+    version: 1
+    metadata:
+      name: matrix-brand-default-app
+      labels:
+        blueprints.goauthentik.io/instantiate: "true"
+    entries:
+      # Root cause behind several dead ends: an authenticated user hitting "/"
+      # with no other destination (e.g. after logging in mid-way through the
+      # TOTP/WebAuthn setup flows) falls back to Brand.default_application: if
+      # unset, that's /if/user/, which type=external Matrix accounts can't
+      # open. Only affects the bare "/" fallback - explicit URLs like
+      # /if/admin/ are unaffected, so internal/staff access is unchanged.
+      - model: authentik_brands.brand
+        state: present
+        identifiers:
+          domain: authentik-default
+        attrs:
+          default_application: !Find [authentik_core.application, [slug, matrix]]